Just more than half of the 4,725 teachers called in absent for Denver's first strike in 25 years. Some students crossed picket lines to get to class as schools remained open with administrators and substitute teachers.

DPS said it typically serves 31,600 breakfasts and that dropped to 22,796 on Monday. Bus ridership fell from 10,206 to 3,300.

DPS said teachers' absences were lower at high-poverty schools. Absences at the 30 high-poverty schools was 44 percent compared to 56 percent across the district.

At East High School, students danced and chanted in the hallways as they walked out to demonstrate to support their teachers. Other students joined hundreds of teachers and union members in a march past the Denver City and County Building.

Hundreds of teachers rallied on the west steps of the State Capitol on Monday afternoon to demand livable wages.

After 15 months of failed negotiations between the district and the DCTA, teachers walked out.

The latest offer from the district is still $5.5 million short of what teachers are requesting.

"We are sending a strong message to the district that they can't do our jobs with us," said Rob Gould, the lead negotiator for the DCTA.

DPS said it has bumped its initial offer of a 10 percent increase in base pay to 10.9 percent, but the union said that should be higher.

The district said it is feeling the effects of the strike after just one day.

"It's a problem," superintendent Susana Cordova said. "I want to be really clear. It's a problem today. It is a problem for our kids not to have their teachers in class and so I want to get this done now."

The strike is affecting about 71,000 students and comes about a year after West Virginia teachers launched the national "Red4Ed" movement with a nine-day strike in which they won 5 percent pay raises.

The dispute is over the school district's incentive-based pay system.

The district gives bonuses ranging from $1,500 to $3,000 a year to teachers who work with students from low-income families, in schools that are designated high priority or in positions that are considered hard to staff, such as special education or speech language pathology.

The union is pushing to lower or eliminate some of those bonuses to free up more money for overall teacher pay.

The district sees the disputed bonuses as key to boosting the academic performance of poor and minority students.

Teachers say the reliance on bonuses leads to high turnover, which they say hurts students, and that spending money on smaller class sizes and adding support staff, like counselors, is the best way to help disadvantaged students.

Some teachers became ineligible for bonuses after their schools lost their official low-income status because parts of the city are undergoing gentrification.

The district has proposed raising starting pay from $43,255 to $45,500 a year. That's $300 a year less than the union's proposal, which would add $50 million a year to teacher base pay, according to union officials.

Negotiations will resume Tuesday morning.

The strike happened after Gov. Jared Polis' administration decided last week not to get involved, believing administrators and teachers were close to an agreement.

However, Polis, a Democrat, said the state could intervene and suspend the strike for up to 180 days if the walkout drags on.

The state does not have the power to impose any deal on either side. But it can try to help both sides reach a deal and can require them participate in a fact-finding process.